It is known that thiopyrylium dyes or pyrylium dyes can be used for various purposes. For example, they can be used for direct positive photographic silver halide emulsions as electron acceptors as described in Japanese Patent Application (OPI) No. 40,900/71 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and also they are useful as spectral sensitizers for photoconductors, in particular as spectral sensitizers for organic photoconductors as described in U.S. Pat. Nos. 3,141,700, 3,250,615, and 3,938,994, and in Research Disclosure, 19321, page 5 (Nov., 1977).
Photoconductors sensitized with thiopyrylium dyes or pyrylium dyes can be used for various purposes as disclosed in the above-illustrated patents but they are particularly important for xerography and Electrofax-system electrophotography.
However, conventionally known thiopyrylium dyes are unsatisfactory for obtaining a photoconductive composition having a sufficiently high sensitivity to light in the red region (600-700 nm) and hence it has further been desired to improve the sensitivity of photoconductive compositions.
Since conventionally known thiopyrylium compounds have a substituent which is conjugatable to the thiopyrylium nucleus or a condensed ring of a conjugatable nucleus, such compounds have a side absorption near the blue region, i.e., a wavelength of 400 nm and a photoconductive composition prepared using such a thiopyrylium compound frequently results in an undesirable yellow-mixed colored photosensitive composition, and the composition has a disadvantage that the yellow-mixed coloration further increases when the composition is exposed over a long time, because dyes included in the composition decompose by the exposure over a long time to decrease a red-region absorption and to increase, on the contrary, blue-region absorption.